1. Field of the Invention
The invention relates to hybrid electric vehicle powertrains in which an electric motor is a source of reverse drive torque.
2. Background Art
A hybrid electric vehicle powertrain with split power flow paths includes an electric power source and a mechanical power source, such as an internal combustion engine. A high voltage traction motor, a battery and an electrical generator are electrically coupled. The engine and the traction motor are drivably connected to vehicle traction wheels through power transmission gearing.
A powertrain having a configuration with split power flow characteristics is disclosed in U.S. Pat. Nos. 6,991,053 and 7,285,869, which are assigned to the assignee of the present invention. That configuration includes a planetary gear system that establishes a divided torque delivery path from the electrical and mechanical power sources to the vehicle traction wheels. The planetary gear system includes a ring gear drivably connected to the traction wheels through transmission gearing, a sun gear drivably connected to the generator and a carrier drivably connected to the engine. The generator, the motor and the battery are electrically coupled.
Positive engine torque applied to the carrier during forward drive of the vehicle in a split power delivery mode is in a direction that enhances the drive torque from the motor at the vehicle traction wheels as shown in equation (1) below. Conversely, during reverse drive, the drive torque from the motor at the vehicle traction wheels is in the direction opposite to the torque in forward drive. Engine torque thus reduces the net driving torque at the vehicle traction wheels. A particular example of this is shown in equation (2) below when Tmotor equals −200 Nm. It is seen from equation (2) that any positive engine torque will reduce the absolute value of wheel torque during reverse drive.
Engine torque output during reverse operation is minimized to maximize reverse drive performance. If possible, all reverse drive torque is obtained from electric power provided to the motor by the battery. However, if the battery cannot provide sufficient electric power to meet the driver's request for reverse torque, then the engine must be used to drive the generator to generate electric power to charge the battery.
Positive driving torque developed by the engine reduces the net available wheel torque in reverse as demonstrated by the following steady-state equation:TWheel=K1(Tmotor+K2Tengine),  (1)where:                K1 and K2 are positive mechanical gear ratios, and        Tmotor=motor torque, which is positive in the forward direction and negative in the reverse direction, and        Tengine=engine torque.        
During reverse drive, for example, Tmotor might equal −200 Nm. Wheel torque Twheel then would equal:TWheel=K1(−200 Nm+K2Tengine)  (2)
Because of cold operating temperatures, or high operating temperatures, or age, or battery state-of-charge limits, the high voltage battery may not be capable of meeting a driver's request for reverse torque at the traction wheels. The engine, therefore, must be used to generate electric power for use by the electric motor to power the traction wheels. In this situation, equations (1) and (2) demonstrate that the available reverse wheel torque then will be reduced.